Golf shafts are typically manufactured with a predetermined stiffness or flex. The term “stiffness” refers to a shaft's flex characteristics. A golfer can choose among golf shafts of different stiffnesses produced by various manufacturers. However, one manufacturer's “regular” flex could be another manufacturer's “stiff” flex, and vice versa.
It is well known that the stiffness or flex of a golf shaft plays a fundamental role in the behavioral characteristics of a golf club. The stiffness of a golf club shaft and the so-called kick point affect, for example, the launch angle or trajectory of the ball and the distance of ball travel. A shaft can have a high kick point (maximum bend closer to the grip), or a low kick point (maximum bend closer to the club head) or a kick point at a location there between Prevailing weather conditions can also affect the optimum stiffness for a club shaft. For example, on a windy day, a golfer might choose to use a club head associated with a shaft that has a low or a high stiffness in order to better control the trajectory of the ball. Or an older golfer may desire to use a golf club with a more flexible shaft than a stiff shaft with the goal of having the ball travel farther.
Various proposals to provide a variable stiffness for a golf club shaft (or even a fishing pole) have previously been made that involve using a hollow shaft charged with a gas or liquid fluid that can be pressurized. Increasing the fluid pressure in the shaft increases the shaft stiffness. Such pressurizable shafts are illustrated, for example, by Menzies U.S. Pat. No. 1,831,255, Sears U.S. Pat. No. 2,432,450, Busch U.S. Pat. No. 3,037,775, Burrough U.S. Pat. No. 4,800,668 (a fishing rod), Simmons U.S. Pat. No. 5,316,300, Koch et al. U.S. Pat. No. 5,540,625, and Painter U.S. Pat. No. 5,632,693.
So far as is known, these fluid charged, variable stiffness, hollow shaft structures of the prior art suffer from the problem that a change in the external environmental temperature inherently causes a significant change in the internal shaft pressure and thus in the shaft stiffness. The change in shaft thickness occurs because temperature changes cause pressure changes in the shaft fluid. Changes in shaft stiffness can dramatically affect the performance characteristics of a golf club. In view of a shaft stiffness change caused by an external temperature change, the performance characteristics of the shaft will change. Outside environmental temperature changes can occur relatively rapidly not only from day to day, but even during a single round of golf. A golfer's expectation that the fluid charged shaft of one of his golf clubs maintains a constant flex characteristic is no longer true after a change in the temperature.
In order for a golf club whose stiffness is regulated by a fluid in its hollow shaft to be practical, the shaft needs to have a stiffness that not only is adjustable but also is able to maintain a chosen stiffness automatically in response to changes in exterior environmental temperature. The present invention overcomes the inability of prior art fluid-filled shafts to maintain a chosen stiffness environmental temperature changes. A shaft is provided which is stiffness adjustable and automatically maintains a selected stiffness regardless of exterior temperature changes.